Updating LLVM in Rust

# Updating LLVM   Rust supports building against multiple LLVM versions: * Tip-of-tree for the current LLVM development branch is usually supported within a few days. PRs for such fixes are tagged with `llvm-main`. * The latest released major version is always supported. * The one or two preceding major versions are usually supported. By default, Rust uses its own fork in the [rust-lang/llvm-project repository]. This fork is based on a `release/$N.x` branch of the upstream project, where `$N` is either the latest released major version, or the current major version in release candidate phase. The fork is never based on the `main` development branch. Our LLVM fork only accepts: * Backports of changes that have already landed upstream. * Workarounds for build issues affecting our CI environment. With the exception of one grandfathered-in patch for SGX enablement, we do not accept functional patches that have not been upstreamed first. There are three types of LLVM updates, with different procedures: * Backports while the current major LLVM version is supported. * Backports while the current major LLVM version is no longer supported (or the change is not eligible for upstream backport). * Update to a new major LLVM version. ## Backports (upstream supported) While the current major LLVM version is supported upstream, fixes should be backported upstream first, and the release branch then merged back into the Rust fork. 1. Make sure the bugfix is in upstream LLVM. 2. If this hasn't happened already, request a backport to the upstream release branch. If you have LLVM commit access, follow the [backport process]. Otherwise, open an issue requesting the backport. Continue once the backport has been approved and merged. 3. Identify the branch that rustc is currently using. The `src/llvm-project` submodule is always pinned to a branch of the [rust-lang/llvm-project repository]. 4. Fork the rust-lang/llvm-project repository. 5. Check out the appropriate branch (typically named `rustc/a.b-yyyy-mm-dd`). 6. Add a remote for the upstream repository using `git remote add upstream https://github.com/llvm/llvm-project.git` and fetch it using `git fetch upstream`. 7. Merge the `upstream/release/$N.x` branch. 8. Push this branch to your fork. 9. Send a Pull Request to rust-lang/llvm-project to the same branch as before. Be sure to reference the Rust and/or LLVM issue that you're fixing in the PR description. 10. Wait for the PR to be merged. 11. Send a PR to rust-lang/rust updating the `src/llvm-project` submodule with your bugfix. This can be done locally with `git submodule update --remote src/llvm-project` typically. 12. Wait for PR to be merged. An example PR: [#59089](https://github.com/rust-lang/rust/pull/59089) ## Backports (upstream not supported) Upstream LLVM releases are only supported for two to three months after the GA release. Once upstream backports are no longer accepted, changes should be cherry-picked directly to our fork. 1. Make sure the bugfix is in upstream LLVM. 2. Identify the branch that rustc is currently using. The `src/llvm-project` submodule is always pinned to a branch of the [rust-lang/llvm-project repository]. 3. Fork the rust-lang/llvm-project repository. 4. Check out the appropriate branch (typically named `rustc/a.b-yyyy-mm-dd`). 5. Add a remote for the upstream repository using `git remote add upstream https://github.com/llvm/llvm-project.git` and fetch it using `git fetch upstream`. 6. Cherry-pick the relevant commit(s) using `git cherry-pick -x`. 7. Push this branch to your fork. 8. Send a Pull Request to rust-lang/llvm-project to the same branch as before. Be sure to reference the Rust and/or LLVM issue that you're fixing in the PR description. 9. Wait for the PR to be merged. 10. Send a PR to rust-lang/rust updating the `src/llvm-project` submodule with your bugfix. This can be done locally with `git submodule update --remote

This section describes how to update the LLVM version used by Rust, detailing the supported versions and the process for backporting changes from upstream LLVM. It outlines the procedures for backporting changes when the upstream version is supported and when it's not, including forking the rust-lang/llvm-project repository, merging or cherry-picking commits, and updating the rustc submodule.